April 3, 2008

Study: Humans Drove Final Nail into Mammoth Coffin

Humans may have struck the final blow that killed the
woolly-mammoth, but climate change seems to have played a major part in setting
up the end-game, according to a new study.

Though mammoth populations declined severely around 12,000
years ago, they didn't completely disappear until around 3,600 years ago.
Scientists have long debated what finally drove the furry beasts
over the edge. Researchers led by David Nogues-Bravo of the National Museum of
Natural Sciences in Spain used models of the climate, as well as models of woolly-mammoth
and human populations, to study the relative importance of various factors
leading to the mammals' demise.

The scientists published their results in the journal PLoS Biology.

The team found that the brunt of the damage done to mammoths
was due to Earth's
warming weather
around 8,000 to 6,000 years ago. Since Earth was coming out
of a glacial period at that time, temperatures were climbing and recasting the
planet's landscape, and the mammoth's preferred habitat, steppe tundra, was
vastly reduced.

The researchers calculated the temperature window in which
mammoths can survive by matching known fossil specimens with climate models. They
determined the temperature at the time each mammoth specimen lived and combined
the data to get an overall picture of the animals' preferred climate range.

The team found that by 6,000 years ago, mammoths were
relegated to 10 percent of the habitat that had previously been available to
them 42,000 years ago when the glaciers were at their largest size and greatest

But climate doesn't seem to explain the entirety of the mammoth's
. These hardy animals had survived, barely, a previous
interglacial period of planet warming around 126,000 years ago. Scientists have
found some fossil bones from this time, so climate change didn't completely
knock out mammoths then.

One difference between that first interglacial period and
the second one during which they actually died off was the presence of humans. Around
6,000 years ago when the climate warmed in North Eurasia where mammoths lived,
our ancestors were able to move in to the region. Once there, they might have
hunted the already weakened population of mammoths to oblivion.

"During the [earlier] interglacial period, climates
were fairly warm, so why didn't [mammoths] go extinct then?" said Persaram
Batra, a climate modeler at Mount Holyoke College in Massachusetts, who worked
on the study. "It could be because humans weren't there. Mammoth
populations were so sparse, that if there had been humans, maybe they would
have gone extinct."

The researchers calculated that by 6,000 years ago, an
optimistic estimate of mammoth numbers would mean humans would only have to
kill one mammoth each, every three years, to push the species over the brink. A
more pessimistic calculation figures that even if one mammoth per human were
killed every 200 years, they would still die off.

"This paper argues that climate change would have
reduced the size of the habitat for the mammoths to the point where hunting
could have extinguished them," Batra told LiveScience. "We're arguing that it's sort of a combination.
Climate change probably didn't do it completely, but it made their life so
precarious that humans could come in and kill them off."